RPMs too low at WOT

[DixieBoatN]

I have a 1985 V150 XR2. I recently put a new foot on the motor due to losing the gears in my old one. Since I put the new foot on everything has been great. I was turning approximately 5500 RPMs at WOT. Not too long ago I went out on the lake one day and out of nowhere the motor would bog under throttle and die. I limped it back in and changed the water separating fuel filter and put a new inline filter on it. The bogging problem went away.

Ever since then I have been only turning 4500-4800 RPMs at WOT. It won't turn more than that and my top speed is down by about 10mph. I don't think the problem is fuel related because pumping the bulb does not do anything for it. I replaced both fuel filters again too because I noticed a bunch of stuff in the inline filter, but the problem persists.

The day it bogged down on me I tinkered with the throttle cable and it was idling high afterward. I have since corrected the idle problem by correctly adjusting the throttle cable again. The WOT problem persisted.

I have bypassed the oil injection system but don't think that would cause any issues and I don't recall the problem starting immediately after I bypassed it.

I had some problems with the arm on my throttle/shifter due to splines being worn on the shaft where the arm connects to the housing, but I temporarily corrected this by tightening the set screw until I can find a good set of controls to replace mine with. I don't think the controls are the problem, but I am not ruling anything out.

I have been looking for a factory service manual but have not been able to find one for the XR2... not even on ebay.

I am a pretty good automotive mechanic, but I am new to the 2 cycle world.

I suspect a problem with the ignition system, but have not found any fouled plugs and do not have the device to test the power packs. It seems and sounds like a retarded ignition, but I may be completely off of my rocker. I do notice when I go from idle to WOT as it runs up I hear it (not sure of RPMs because I have not watched tach during this) run the revs up and then drop back a few revs before topping out completely.

I did have a repeat of the bogging problem this past Tuesday when I was way out on the lake trolling and a storm started rolling in so I ran it wide open for longer than normal trying to get back to the dock. About 3/4 of the way back to the dock(25-30 min ride total) it started bogging and I had to back off of the throttle to keep it from dying. It died and after re-cranking a couple of times and babying the throttle, I was up and running again.

Any ideas on what/where to look are appreciated! If you know where I can buy a factory service manual without breaking the bank, I'd appreciate that too.

 

[DixieBoatN]

Re: RPMs too low at WOT

Bueller? Bueller?

 

[cell]

Re: RPMs too low at WOT

I am a pretty good automotive mechanic, but I am new to the 2 cycle world.

Treat it as a auto.Fuel,air,spark,and compression.If one drops it wont run as it should.

Are you using the same prop as before the lower end change?

 

[j_martin]

Re: RPMs too low at WOT

Given the time frame you stated, there's a chance you did some serious mechanical damage when it leaned out. That 2 stroke runs about a 2 million BTUH fire inside it at WOT. It don't like lean, and can grenade in seconds when starved.

Check compression first.
Check for spark on all 6
Check for cylinder strength by disconnecting one spark wire at a time while idling on the cuff.

Then 'cause ya been messing with the throttle (timing) arm, check your timing and link and sync.

Put a 10 lb gauge on and check the fuel pressure under all conditions. Find and fix the fault if it goes below 1 lb at any time. Should be about 5 most of the time.

If good, go through all the carbs, and look for a small passage plugged by the same gunk that caused the original problem.

Go get a manual from your mercury dealer. would be about 30 bucks or so. It's a hellova lot cheaper than grenading a power head by guessing at an adjustment. Alternatively, look up the part number for the manual on the parts list for your engine at a site like mercruiserparts.com (they have outboard parts) and then scan eBay and craigslist for one.

hope it helps
John

 

[DixieBoatN]

Re: RPMs too low at WOT

cell: I am using the same prop.

j_martin:I just checked compression this morning. All is good, so hopefully nothing is grenaded. With engine cold, all checked out to 100-110psi. It is the exact same as it was when I bought the thing a few months ago (and problem was not happening then).

I have Clymer's manual and will do lync-n-sync procedure per that manual. If I have to, I will pay the $70-$80 for factory service manual from Mercury parts dealer. I just hoped to not have to since this was older motor, so I figured there would be other used sources.

I checked for spark on all 6 awhile back when this first happened, but I will check it again.

If I disconnect a plug while it is idling on the cuffs, I should hear it stumble, correct? If it does not stumble, I have a bad plug/coil/etc... correct?

I just purchased a cheap-o fuel pressure gauge from the auto parts store and plan to plumb this in at the fitting that connects to the motor so I can look back at it to verify fuel pressure.

Thanks for the help.

 

[Starcraftguy1]

Re: RPMs too low at WOT

I think the carbs have some kind of debris.Or the fuel pump.Check them out before messing with important things.Your fuel filter had debris in it,sounds like that to me.

 

[Searay 150merc]

Re: RPMs too low at WOT

My boat was running the same way. Rebuilt carbs and fuel pump. fixed the problem.

 

[DixieBoatN]

Re: RPMs too low at WOT

I was doing some more experimentation today and I was able to determine a couple of things.

1. I used a timing light and checked all cylinders with it on the cuffs and I am getting fire on all cylinders.
2. When I unhooked each spark plug with it idling on the cuffs I heard the engine run rougher except on cylinders 4 and 5. Cylinder 4 is on one side of the motor and cylinder 5 is on the other. I am thinking this does not mean that there is a problem with the power packs.

I am also thinking that since I am getting some spark on these cylinders, it is not stator/rectifier.

This looks like a coil problem. My Clymers manual states that I need a DVA meter to test the coil. Is this correct? I can not just test for resistance across primary and secondary windings like on an automotive coil?

 

[wellsc1]

Re: RPMs too low at WOT

Until reading the last post, I would have stressed the fuel system from fuel tank to carbs. But you may be on to something with two cylinders maybe not firing. If that becomes a dead end, go back to examining the fuel system with possible carb overhaul.